1. Field of the Invention
The present invention relates to an apparatus suitable for molding of flat, close-tolerance, thermosetting, graphite resin plates useful as precursors for carbonizing to vitreous carbon plates usable as separator plates in batteries or other electrochemical cell devices. More specifically, it relates to the molding of such plates which are very thin but of considerable dimensions in width and length. Still more specifically, it relates to a mold design having an inverted cavity arrangement in an upper portion of the mold permitting removal of the molded product on a flat sheet lower portion of the mold.
2. Description of the Prior Art
There are many prior patents relating to the composition of various thermosetting graphite resin blends useful for molding to dense graphite articles as described by U.S. Pat. Nos. 3,907,950, 3,969,124 and 3,624,569. However, little teaching exists relating to actual mold construction for shaping this material into uniformly thin, flat, thermoset, close-tolerance sheets or plates.
Rempel (U.S. Pat. No. 2,367,525) shows a design of a movable platen supported on rollers in sockets on the underside of the platen to permit the feeding of preforms into and removal of product from a compression or transfer mold. Most of the details are directed to the roller system for the massive, heavy platen. There is no indication of the location of the mold cavity and no indication that it could be in the upper section of the two-part mold.
Buonaiuto (U.S. Pat. No. 3,676,033) describes compression and transfer molding apparatus for making thermoplastic cellular products using a performing station to convert thermoplastic expandable beads by heat to a fused preform and then transferring this preform to the final forming station. This patent does not teach the making of flat plates of filled thermosetting materials. Neither Buonaiuto nor Rempel teach the use of an inverted cavity in an upper mold portion which closes onto a flat, movable plate.
As U.S. Pat. No. 4,301,222 (Emanuelson et al) reveals, separator plates for use in batteries and other electrochemical cell devices need to meet high standards in terms of many different characteristics including hydrogen permeability, corrosion resistance, electrical resistivity, thermal conductivity strength and electrolyte absorption resistivity. As this patent reveals, plates are preferred to be less than 0.050 inch thick. It also reveals that the largest such plates to have been molded have been 25.times.27 inches.
This same patent also acknowledges that "It has been desirable to make these plates thinner for improved electrical and thermal conductivity and for more economical and more versatile fuel cell configurations. This makes them even more difficult to fabricate with the requisite strength and impermeability." It also reveals desirable physical properties of such plates. Sufficient dead weight is applied vertically to each stack of a number of such plates to maintain plate flatness during the carbonizing period that converts the graphite-resin plate to vitreous carbon. The plate edges in the stack must be flush with each other and the plates must be fully supported to maintain separator flatness.
In certain industries, the need has arisen for uniformly thin plates having width and length of large dimensions with little or no waviness or bowing. Present mold designs have not been able to satisfy these needs. Out-of-plane deformation at the edges in applictions such as for separator plates is especially to be avoided. Moreover, when such plates are lifted out of a mold, it is almost impossible to avoid stresses and possible deformation at the areas by which said plates are lifted from the mold. Furthermore, when such plates are lifted out of a mold, the edges of the product are in close contact with the sidewalls of the mold in such a manner that friction against or sticking to the mold sidewall often causes the edges of the product to be distorted or wavy as compared to the inner areas of the plate.
In molding flat, thin, close-tolerance graphite-thermosetting resign plates (resin-plates) for after firing to vitreous carbon, plates have been previously molded through conventional compression molding techniques using positive type molds wherein the plate cavity is formed by means of a cavity block set between rails held in place on the lower heating platen. The force or plunger is attached to the upper heating platen. To form close-tolerance, flat, thin, precursor, graphite-thermosetting resin plates, experiments with positive type molds have been attempted. The resulting precursor graphite-filled thermosetting plates are relatively flaccid at the time of press opening and easily subject to distortion if lifted.
With such positive type molds, problems encounted in attempts to produce uniformly flat, thin, close-tolerance, large size plates (48".times.48".times.0.040"), have included shorts, warpage, center-bowing and plunger tilting. Further, with a positive mold, flashing is forced to rise perpendicularly out of the mold plane and around the mold plunger, resulting in part hangup in the cavity. Cycling often needs to be interrupted while the parts are freed and the mold cleaned. Forces exerted on the part to free it during the critical period following press opening often warps the hot flaccid curing part necessitating rejection of the part since critical tolerances are not obtained.
Such conventional flash type molds are impractical since these molds contain the cavity on the lower heating platen. Ejection of large size hot flaccid flat parts generally induce plane deformations, warpage and deviation from critical tolerances. Frequent out-of-plane deformations result from deflashing operations and from attempts to free sticking hot flaccid parts upon press opening. These cause defective plates, resulting in a large percentage of rejected plates. Plates free of out-of-plane deformations and without edge waviness are essential in certain applications such as batteries and electrochemical fuel cell devices.